1. Field of the Invention
The present invention relates to devices to supply accurate time period information to position sensors which require the accurate measurement of a time period and the methods for accurately measuring such time periods at high resolution.
More particularly, the present invention relates to sensors based on the principle of magnetostriction which require accurate measurement of time periods and novel methods using magnetostrictive position sensors to make high resolution time period measurements without increased power requirements.
2. Description of the Prior Art
In general, magnetostrictive position sensors include a ferromagnetic delay line, which is occasionally called a xe2x80x9cwaveguidexe2x80x9d. A pulse generator supplies a current pulse to the delay line, generating a magnetic field which surrounds the delay line. A remote and movable position indicating magnet is positioned along the delay line. The magnetic field of the position magnet disturbs the magnetic field generated by the current pulse.
The interaction between the magnetic field of the position magnet and the magnetic field induced by the current pulse causes a strain or mechanical reaction within the delay line. This strain induced reaction force within the delay line is propagated along the length of the delay line as a torsional acoustic wave.
A detector, called a xe2x80x9cmode converterxe2x80x9d, is typically attached to one end of the delay line. This mode converter detects the passage of the torsional acoustic wave and converts it into a representative electrical signal.
The time delay period from the excitation of the waveguide to the reception of the corresponding acoustic wave at the mode converter indicates the location of the position magnet along the length of the delay line.
A variety of time measurement or intervalometer techniques have been used to convert the time period information into a position indicating signal.
U.S. Pat. No. 3,898,555 to J. Tellerman discloses a fixed frequency oscillator to initiate the excitation pulses to the delay line. The returned acoustic signal, in conjunction with the fixed frequency oscillator, develops a signal which is xe2x80x9cpulse width modulatedxe2x80x9d by the position of the magnet along the delay line. An integrator converts the pulse width modulated waveform to a DC voltage level which forms the transducer output.
U.S. Pat. No. 4,721,902 to J. Tellerman et al. discloses inter alia, a method to convert the pulse width modulated signal into a digital value. The patent teaches the use of a conversion counter to collect xe2x80x9ccountsxe2x80x9d from a conversion oscillator during the xe2x80x9conxe2x80x9d time of the pulse width modulated signal.
U.S. Pat. No. 5,070,485 to D. Nyce discloses an analog averaging technique to improve resolution at low power, but offers a much slower response time.
Magnetostrictive position sensors of this type are used in the measurement and control industry. They find use in machine tools, in robotics, as liquid level indicators, as well as other applications. In many of these applications, high speed and high resolution are both important.
In the prior art, simultaneous high speed and high resolution measurements for magnetostrictive sensors required a high power, high frequency clock (≈100 Mhz). Thus, it would represent an advancement in the art to enable high resolution measurement without the need for high precision clocks.
It is also known in the art to use lower precision clocks to obtain higher precision time measurements. See EPO Application Serial No. 0508232A2.
In contrast to prior art magnetostrictive measurement systems, the present invention provides a novel method for measuring a time period for a magnetostrictive device or other time interval sensitive devices or other devices which may be arranged to be time interval sensitive, such as an RTD measuring device.
The method of this invention includes measuring a coarse count to approximate a time period to be measured using a low frequency clock (coarse clock) and measuring a fine count of the time period to be measured (ie., resolve the less significant bits) using a pulsed high frequency clock (fine clock). The fine count is then added to the coarse count to obtain a total high resolution representation of the time period.
The fine counter, clock or counting device does not have to be crystal controlled because its calibration is checked each time by comparing it to the coarse clock, which is crystal controlled. A crystal controlled clock typically requires settling time before it is accurate after being turned on. This method allows the fine counter to be easily cycled without turn-on settling.
The present invention also provides an apparatus for performing the method according to the present invention and includes means for measuring a coarse count, means for measuring a fine count and a means for summing the coarse and fine count and calculating a high resolution representation of the time period.